Photo radio system



Mafch 3,1942.

' R. E; MATHEs Erm.v

yPHQTO RAfDId SYSTEM Filed April 16, 1940 11 'sheets-sheet 1` ...AAA-

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ATTORNEY.

R. E. MATHES .ETAL

PHoTQRAnIo sYsfrEM File@ April 16, 1940' March 3, 1942.

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ATTORNEY.

l March 3,v 1942.

R E. lMATl-lEs ET AL PHOTO RADIO SYSTEM Filed April-1e, 1940 4 sheets-sheet 5 4 Sheets-Sheet 4 R. E. MATHESETAI.

PHOTO RADIO SYSTEM Filed April les, 1940 March 3, 1942.A

Patented Mar. 3, 1942 PHOTO RADIO SYSTEM melma E. Mathes, westaeid, N. J., and warren H. Bliss, Orono, Maine, assign'ors to Radio Corporation of America, a corporation of Delaware Application April 1t, 1940, serial No. 329.990

45 Claims. (Cl. P18-6.7)

This invention relates to signalling systems and is shown in connection with the sending of pictures, though by way of example only, since it may be used in other forms of communication.

In photo radio transmission, particularly over long distances, it is desirable to transmit the signals in telegraphic form, that is, by interrupted waves which appear as individual current pulses after detection at the receiver. For perfect recording these pulses should appear at the receiver exactly as sent out by the transmitter.

This seldom happens over any considerable.

length of time as the pulses are materially changed by fading, echoes and other multipath effects. A particularly objectionable feature is the addition of elongations or tails to the pulses by a combination of signals arriving at slightly different times over dierent paths.

To overcome the defects heretofore experienced, we have devised a system in which it it an object to communicate by phase modulation and convert the signals at the receiver into amplitude or other modulation.

Another object is to transmit the signals in the form of pulses of constant time length and of varying phase.

Another object is to block out at the receiver all of the received pulses except the initial part.

Another object is to block out multipath additions to the signals.

Another object is to utilize at the receiver only the initial part of the received phase pulses and to change the phase modulation into voltage or current variations.

Other objects will appear in the following description, reference being had to the drawings in which:

Fig. 1 is a diagrammatic illustration of the transmitter.

- Fig. 2 is a diagram of the limiter and wave shaper at the receiver. i

Fig. 3 is a diagram of the impulse integrator at the receiver.

Fig. 4 is a diagram of the synchronous frequency source, the phase shifter and the sawtooth oscillator used at the receiver.

Referring to Figure l, the transmitting systemeonsists of a scanning device generally indicated at I. This may be of various types but we have indicated by way of .example a cylinder 2 on which the object to'be scanned is clamped by a clamping device 3 which also may be of any construction. The cylinder is rotated at a constant speed by any means, a synchronous motor 4 being indicated for purpose of explanation.

Thismotor may be run from the constant frequency supply source 5 of any kind, various types being well known in the art. (See article by Callahan, Mathes and Kahn on Time Division Multiplex innadio 'reiegr'aphic Practice, I. R. E.,

vol. 26, No. 1, page 35, January, 1938.)

As usual the cylinder 2 moves longitudinally in Fig. 1 in respect to the scanning apparatus consisting of anlilluminating device 6` and alight focusing and photo electric element designated The relative longitudinal travel between the cylinder and the devices 6 and l is diagrammatically illustrated as being produced by a screw axle 8. During each revolution of the drum 2 a spiral path 'is traced around the object and continued rotation finally scans the entire j Once inv each revolution the clamping object. means 3 passes under the photo-electric focusing element. This clamp\will have uniform light value,` preferably totally black, so that ,there will be a sries of pulses, for example, about forty in our experiments, that are of uniform amplitude andI phase. These are utilized for adjusting the receiving recorder with the scanner at the transmitter as later explained in detail.

As Well understood,`the scanning of the object in cylinder 2 will produce a voltage and current in resistance 9 of amplitude proportional to the light values of the elements scanned. The amplifled varying potentials appear in the common leg of the grid circuit I0 of the push pull modulating tube into' which is introduced the subcarrier frequency from source II. As is well known, there will appear in the output circuit I2 of the push pull modulator a voltage varying in amplitude with the modulations appearing in resistance 9.

As is well known, the voltage in the output I2 can be made to vary inversely with the modulation in resistance 9 by reversing the grid circuit connections to the resistance 9 and preferably reducing the negative grid bias I3. That is, the light and dark elements in the object can be made to produce increase and decrease of voltage, or vice versa, in the output circuit I2.

The modulated sub-carrier frequency in output I2 is fed into a rectifier I4 so that D. C.

modulations appear in load resistance I5. The

voltage in this resistance is filtered as at I5' and applied to the grid of triode I6 having resistance I1 in the plate supply lead and a load resistance I8 in the cathode lead connected to ground which is also the minus plate supply terminal. f,

The output voltage in resistance I8 is adjustably connected between the grid and cathode of a gas triode I9 known generally as a thyratron. The cathode connection is made from ground through resistance 2| and resistance 22. Gas triode 29 has its output circuit connected in parallel with that of gas triode I9 with al commutating condenser 24 connected between the plates of the two tubes. The gas triodes have resistances 25 and 29 in the plate leads.

Gas triode 23 has resistances 21, 29 in the cathode lead and a resistance 29 shunted by a condenser 991s adjustably connected to the resistance 28 to furnish the proper negative bias. Resistances 2| and 29 have a by-pass condenser 3| and resistances22 and 21 have by-pass condensers 32 and 59, respectively.

Vacuum tube 94 has its output circuit connected in parallel with that of vacuum tube I9 through resistance I1. Condenser 99 is connected between the grid and cathode of tube 94. The grid andcathode of this tube are also connected through resistances 2|, 99 and the gas diode 31, the grid beingl connected to the cathode terminal thereof. The grid and cathode of vacuum tube 94 are also connected together through two element vacuum tube 99 and resistance 22, the grid being connected to the cathode of that tube.

tubel 49 and its load resistance 99 into the grid circuit of amplifier tube I9. This saw-tooth screen frequency is also introduced into resistor 96 through condenser 5I and transformer 99.

The saw-tooth generator consists of a gas triode 49 having a condenser 4I connected between its plant and cathode. 'Ihis cathode is also connected to the grid of amplifier tube 49 through by-pass condenser 49 and to ground through a current limiting device which, by way of example, we have shown as a pentode 49. Pentode 49 has its plate connected to the cathode of gas'triode 49 and its cathode connected through biasing resistance 41.

With the saw-tooth generator thus described a saw-tooth voltage would be produced, but the frequency would not be constant enough to suit our purposes so we apply a strictly constant frequency to the grid of tube 49 from source 5. This constant frequency may be equal to the desired saw-tooth frequency or any multiple thereof.

By the term saw-tooth generator, as used herein, we mean a vgenerator having a voltage wave with an inclined side and a substantially vertical side for each cycle. This wave form, simulating a right angle triangle. has the advantage that phase modulations may be obtained in the transmitted pulses substantially over the entire cycle. With a screen generator having each cycle of the voltage wave simulating an isosceles triangle the phase modulations could vary, at the most, over only half of the cycle. There is, therefore, an

circuit of-triode I9. 4

In producing the saw-tooth voltage condenser 4I is charged through current limiting pentode 49 in accordance with the well known voltagetime characteristic. With the D. C. bias in resistance 49 and the plate voltage applied through resistance 49, a point will be reached where the rising voltage of the condenser will be able to cause the gas triode to strike and thus discharge the condenser relatively instantaneously. The time required for charging the condenserl jto striking voltage is such that the ions in gas tube 49 will diffuse before the condenser is recharged. The frequency thus depends solely upon the time of charge, the discharge being instantaneous for all practical purposes.

Pentode 49, as is well known, has a relatively fiat topped plate voltage-plate current characteristic above a predetermined plate voltage so that the charging current of the condenser 4I is made substantially constant over the charging period by working the pentode on the flat part of its characteristic.

v49 will always strike The action of the saw-tooth generatoras thus far described. would produce say approximately 90 cycles per second, but over an extended period there would be considerable variation. To produce a constant saw-tooth or screen frequency, we apply the constant frequency of the source 5 which will superimpose its frequency on the bias voltage in the grid circuit 1of gas triode 40 and denser voltage being too low to cause it to strike on the eighth and previous cycles. y

It will thus be clear that a constant saw-tooth voltage of desired value appears in load resistor 42. This is amplled by vacuum tube 49 and appears in resistor 99 which applies the saw-tooth voltage to the grid of ampliiler I9 in series with the signal voltage in resistor I5.

Gas triode I9 will not strike until a predetermined voltage is applied to its grid from load resistor I9. Gas triode 29 has such grid bias thatit will strike and continue to conduct until commuted by gas triode I9. i

`,When the combined signal (amplitude modulation) and saw-tooth voltage is suflicient, tube I9 will strike. In this respect it may be said that neither the signal voltage nor the saw-tooth voltage alone issuiilcient tocause tube I9 to` strike, but lthe combination of the two over substantially the entire modulation range and sawtooth cycle will cause it to strike.

As soon as gas triode I9 strikes, it instantaneously lowers the plate voltage of gas triode 29 to a negative value by action of condenser 24 in its inability to discharge instantaneously. This produces a counter voltage between the cathode and plate of triode 29 greater than the voltage tending to drive lthe plate current through it.

. ,When the condenser 24 discharges suiilciently to let theplate of triode 29 go suiiiclently positive triode 29 restrikes, under its constant grid voltage. As soon as triode 29 restrikes, triode I9 will be commuted in an identical manner.

It is desired that triode I 9 will strike only once for each saw-tooth cycle, and to prevent it from Instantly restriking as the saw-'tooth voltage is superimposed on these modulations 1.1 the grid climbs toward its .peak in the same cycle, we kill the signal in the grid circuit by action of tube 34. This acts in the following way:

Before the combined signal and saw-tooth Avoltage cause gas tube I9 to strike. tube I5 has normal amplifying power but as soon as tube I9 strikes condenser 35 is charged by the drop in resistance 22 applied through diode or Fleming valve 33. This applies suicient positive voltage to the grid of tube 34 to cause it to draw heavy plate current. The sudden drop produced in re- .produced from the saw-tooth voltage of leads,

sistance I1 lowers the plate voltage of tube I5 so that it virtually' does not amplify the signal.

'This prevents tube I9 from restriking until the condenser discharges. Condenser 35 cannot discharge through gas diode 31 until the voltage across the diode is augmented to the striking value. This is produced by connecting the sawtooth generator to the resistance 33 through a peaking condenser I' which produces a peaked voltage near the close of each cycle of the sawtooth wave. The combined voltage is sufficient to cause gas'diode 3,1 to strike and discharge the condenser through resistances 33 and 2| at the end of the saw-tooth cycle.' Therefore only. once during each saw-tooth cycle can the combined signal and saw-tooth voltage cause the gas triode I9 to strike.

Since the saw-tooth voltage in each cycle is a linear function of the time, if the modulated signal drops in amplitude the combined signal and saw-tooth voltage will equal the striking value at alater time, assuming a saw-tooth voltage with the vertical side at the terminating end of the cycle. Conversely, with an increase in signal amplitude, the sum o f the two voltages rwill reach the striking value at an earlier time. Therefore, the amplitude variations produced by the scanner are converted into phase modulations. These phase modulations appear as short pulses in load resistance 22 to which the output leads 52 are connected.

The phase modulated output at leads 52 may bev connected to any appropriate transmitting appa# ratus for transmitting a series of high frequency radio signals that might be referred to as signal dots when detected at the receiver. These dots are of uniform amplitude and length as sent out over the air, but as previously explained they usually appear in the receiver as dots or pulses of non-uniform amplitude and non-uniform length due to action of the ionosphere. While travel over long distances usually changes the phase of the pulses in respect to those at the transmitter, the phase of one pulse with respect to another in passing over the same path is, of course, unchanged.

Tne receiving. and detecting apparatus can be of any design, many ofwhich are known, and therefore we have not illustrated them. It is assumed that the pulses have been received 'and detected and are presented to the leads 53, 5I, Fig. 2, as D. C. pulses or dots of the same relative phase as at the transmitter, but generally of non-uniform amplitude and length due to atmospheric effects previously mentioned. These pulses are amplified to the desired amount, the amplier 55 typifying this.

Thev output of the amplifier is fed to a special amplifying tube 55 similar in function to the amplifier I6 at the transmitter of Fig. 1. The output of the amplifier is connected to gas triode 51 by plate lead 53 through condenser 59 and resistance 60 and by-cathode lead 5I and resistance 52. Blocking tube 33, Fleming valve 64, gas

33, 39, by means of condenser 10. The peaked voltage from resistance 'I0' combined with the voltage of condenser 55 discharges the condenser' at adeflnite time through the gas diode 35 and lresistance 1I, as previously described in connection with Fig. 1.

Gas triode 12 has the same function 'as gas Ktriode 23 of Fig. ,1. It is normally conducting while gas triode 51 is normally nonconducting.`

Commutating condenser 13, resistances 14, 15,v 16 and 11, and the associated condensers have similar functions to similar parts in Figrl. In Fig. 2 we have shown an adjustable resistance 1.1 for adjusting the bias of gas triode 51. This is adiusted so that gas triode 51 .will re on the arrival of levery incoming pulse eventhough the amplitudes of the incoming pulse may vary widely due to radio circuit fading. Condenser 55 immediately causes tube 63 tov draw such a heavy current'that the positive bias in resistance 61 causes tube 56 to saturate for the rest of the cycle. Hence multipath additions cannot re-re tube 51. y Y

The output in resistance 62, as also that in resistance 22 of Fig. 1, has a constant amplitude.. as will be readily apparent. Consequently, the

pulses are limited both as to amplitude and time. f

The output leads 13, 19, of the unit in Fig. 2 are tapped off of resistance 62 and as indicated in Fig. 3, these are connected to the input of a pulse integrator and recorder through an electromagnetic switch 30 energized by cam 3| and switch 3l', once each revolution of the recording `drum 32 which is run bysynchronous motor 33 connected by leads 84, 85, to the source of constant frequency shown in detail in Figure 4. This source of constant frequency is of course kept in synchronism, or at a proportional speed, with the similar source at the transmitter, as will be later` described.

The phase of the recorder is adjusted to agree with the transmitter by anyv means such as a cathode ray oscilloscope 33 `well known in the art having its horizontal sweep circuit synchronizedto the constant frequency terminals 34, 35. Its vertical deflection circuit is connected to the signal input 13, 19. f

lInasmuch 'as the clamping means 3 at the transmitter causes a series Vof pulses of uniform phase corresponding to black, the two drums can readily be brought ,into phase if a test strip of white paper or paint is used on the transmitting drum. As the scanner light at the transmitter travels over the white strip during the major part of each drum revolution, pulses willv appear at a constant position on the oscilloscope screen as at 38. However, when the transmitter scanning spot passes clamping block 3 the synchronizing pulses corresponding to black will be sent and since their phase differs from those for white they will appear as at 31 on the oscilloscope screen. The timing of the appearance of.pulses at 31 can be compared with the flashes of light from lamp 39 which occur only when the clamptube and vary the plate impedance.

ing means 90 at the receivingdrum passes under the recording light 9|. One can readily adjust the clutch 92 between the motor 88 and the drum 82 so as to put the two drums in exact phase. This clutch is illustrated as having a pin 9 3 adapted to fit in any of a series of holes 94 in the drum. This is symbolic only as the mechanical clutch may be of various designs.

The leads 19 and 19 of Fig. 3 are connected through the switch 80 to resistance 95 across which ,is tapped a voltage divider consisting of the plate-cathode impedance of vacuum tube 9,6, resistance 98, and rectier 99, the latter being used to permit passage of current only in one direction.. Thel saw-tooth voltage generated by the unit in Fig. 4 applies its voltage to resistance 91 through leads |00 and |0| and transformer |02. Tube 96 is biased by resistance |03 to which the cathode is adjustably connected. This resistance is connected between ground and the positive side of the plate source, as shown. The grid of tube 96 is connected to an adjustable point in resistance 91. One end of resistance 91 is adjustably connected to resistance 95. The output of tube 96 is connected to integrating condenser |04 which applies its voltage to the input of class A amplifier |05. Resistance |06 in the cathode lead furnishes the voltage that operates the recording light 9| or equivalent unit. As well known, the hook-up may be designed to make'either a positive or a negative record.

In the voltage divider, the resistance 98 is constant, but the impedance of tube 96 varies inversely with its grid voltage. Since `condenser |04 is connected across this variable impedance in the voltage divider, a varying part of the signal pulse voltage will be applied to it.

It will be seen that the signal pulses are superimposed on the saw-tooth voltage, one for each cycle in the same phase relation as at the trans-` mitter, because the saw-tooth generator at the receiver (Fig. 4) is kept in synchronism with the saw-tooth generator at the transmitter.

The grid bias of tube 96 is such that the sawtooth voltage cannot cause plate current to pass,

` but the combination of the signal pulse voltage applied through resistance 91 can unblock the The earlier the phase of the pulse, the lower the grid voltage, the higher the impedance and the higher the signal voltage applied to condenser |04. The net result is that at the time the phased pulses come in, the tube conducts and leaves the condenser |04 with a dei'lnite voltage which it keeps until the next pulsel arrives. If that pulse is earlier in phase, the grid voltage will be lower and the impedance drop will boost the condenser voltage to a higher value, which it will keep till the next pulse arrives. If that pulse is later in phase, the grid voltage will be higher and the plate impedance lower. Whereupon, the condenser will discharge through tube 96, then unblocked, to a voltage level commensurate with the grid voltage. The result is that the condenser acquires a voltage having a series of steps in respect to time and having substantially 'constant value in each step between adjacent pulses. This means that the amplier will apply to the recorder a voltage having a value proportional to the phase of the received pulses and consequently proportional to the amplitude modulations ofthe scanner at the transmitter.

Referring now to Fig. 4, the synchronizing unit and phase adjuster for producing a saw-tooth voltage has its input lines |01, |08 tapped oil' of the lines 18, 19, through switch 80 only during the closure of switch 8|' by the cam 9| (Fig. 3) which as shown is when the clamping means 90 is passing under the recording light unit 9|.- This time in the revolution of drum 82 is useless as far as the recording act ls concerned', so it canbe `used for phasing and synchronizing purposeswithout taking up any useful part of the recording time.

The signal input lines |01, |09 are connected to a low pass nlter indicated generally at |09. This filter should have a cut-oil that will eliminate the second harmonic of the signal pulse frequency and pass the fundamental. Resistances I0, are matched input and output impedances, respectively, of the illter. of the lter is connected to the input circuit of class A amplifier tube ||2 and the output of this tube is connected through blocking condenser ||3 to the oscillating unit generally indicated at ||4. This oscillating unit contains a tank circuit ||5 tuned to the screen frequency at the transmitter, that is, to the frequency of the signal.

The output of this tank circuit is connectedto two class A ampliiiers ||6, ||1, by line ||8 and load resistance ||9, respectively. To produce continuous oscillations there must be a feed back to the tank circuit and this is accomplished by leading the output ofvamplifler tube ||6 through cmdenser |20 to the input of amplitude limiter I2 rlhe output of amplier |2| is connected to the tank circuit through condenser |22. Tube 2| has the usual grid leak |29. This puts the leed-back energy in phase with the oscillations.

in the tank circuit. Amplifier tube |2| has a grid voltage-plate current characteristic such that the energy fed back to the tank circuit through resistance |24 is substantially constant. ior further explanation of this form of oscillator, reference is made to the patent of lJames N. Whitaker No. 2,162,520.

Ol' course no oscillating circuit can remain absolutely constant in frequency over an extended time but the one used herein should have a 'stability of about one part in 5x103 in. order for the screen frequency of the transmitter, applied momentarily at the end of each scanning line through condenser H3, to keep it in step. With an oscillating circuit of this constancy the screen frequency (saw-tooth frequency of the transmitter) will keep the frequencyin output resistance ||9 in synchronism therewith. The output vfrom this resistance is amplified by tube ||1 and passed to the phase adjuster generally indicated at |25. This consists of condenser |26 and resistance |21 connected in parallel with a similar condenser |28 and resistance |29 reversely connected to the load 'resistance |99 of transformer |9|. The two resistances |21 and |29 are connected for simultaneous adjustment. These resistances should be equal and each should have several times the impedance value of each of the condensers |26, |28, which latter should have the same value. With this arrangement the adjustment will produce a phase variation of The output of the phase adjuster |25 is connected to a second phase adjuster |32 through class A amplifier |33. Condensers |34, |35, and resistances |36, |81, with simultaneous adjust'- ment of the latter, enables one to obtain 180V change of phase in the input to class A amplifier |38. With this double phase adjuster |25, |82,

The outputsaw-tooth generator indicated at |38 through transformer |40. This saw-tooth generator is identical with that at the transmitter andits various parts have been given the same reference characters. Consequently its construction will be understood from the description of the saw-tooth generator at the transmitter and it need not be repeated. i

The output resistance 50 of saw-tooth generator |39 is connected to the leads 88, 68, oi Fig. 2 and to leads |00, |0|, of-Fig. 3, as the units in both of these figures require a saw-tooth voltage having the same frequency and phase as the saw-tooth screen frequency at the transmitter. It will be clear that the generator ||4 at the receiver will be maintained automatically in synchronism with the generator at the transmitter but thephase oi the saw-tooth voltage is adj justed manually by manipulation of the phase Fig. 3. The phase ofthe saw-tooth wave from generator |39 can then be set so that the incoming pulses for black and white ,bear the proper phase with respect to the saw-tooth wave. Thus the phase adjusters can cause the saw-tooth generator at the receiver to take the correct phase which adjustment will be satisfactory for the entire recording of the picture. Obviously the phase could `be adjusted before starting the picture by transmitting a light wedge" with the same results.

The operation of the receiver is so similar to that of the transmitter that the extended de-l scription of the operation oi the latter will sulce for the former, the variations between the two operations having already been given.

` We have illustrated and described particular devices and circuits but this has been for purposes of explanation of the principles involved and it will be understood that various equivalents may be usedwithout' departing from the spirit of the invention.

It will be apparent that the phase pulses need not be sent out at the leading end of the signals, as they may equally as well be sent out at the lagging end by appropriate reversals of connections at the transmitters and no changes at the receivers.

Having described our invention, what Awe claim is:

l. In photo radio systems, a scanner producing picture modulations, a pair of gas triodes connected together, one normally conducting and one normally blocked, a saw-tooth generator, means to combine the scanner modulations with the output of the saw-tooth generator in the input circuit of the blocked gas triode whereby it fires at the time in each cycle of the saw-tooth voltage that the sum of the amplitudes of the modulation and saw-tooth waves in said cycle first reach the firing voltage, means for blocking the previously conducting gas triode when the other` triode fires, and means to'reblock the last mentioned gas triode and retire the other gas triode before the end of the cycle in which` the blocked triode was red.

2. In photo radio systems, a scanner producing picture modulations, a pair of gas triodes connected together, one normally conducting and one normally blocked. a saw-tooth generator, means to combine the scanner modulations with the output of the saw-tooth generator in the input-circuit of the blocked gas triode whereby it res at the time inl each cycle of the saw-tooth voltage that the sum of the amplitudes of the modulation and saw-tooth waves in said cycle ilrst reach the iiring voltage, means for blocking the previously conducting gas triode when the other triode iires, means to reblock the last mentioned gas triode and reflre the other gas triode before the end of the cycle in whichv the Ablocked triode was fired and means for transmitting the pulses in the output circuit of the second mentioned gas triode.

3. In photo radio systems, a scanner producing picture modulations, a pair oi gas triodes connected together, one normally conducting and one\normally blocked, a saw-tooth generator means to'combine the scanner modulations with the output of the saw-tooth generator in the input circuit of the blocked gas triode whereby it fires at the time in each cycle of the saw-tooth voltage that the sum of the amplitudes of the .modulation and sawtooth"waves in such cycle rst reach the ring voltage, means for blocking the previously conducting gas triode whenl the other triode iires, and means to reblock the last mentioned gas triode'and refire the other gas voltage that the sum of the amplitudes oi the modulations and saw-tooth waves in such cycle iirst reach the firing voltage, means for blocking the previously conducting gas triode when the other triode ilres, means to reblock' the last mentioned gas triode and reflre the other gas triode before the start of the next cycle of the saw-tooth voltage and means for transmitting the pulses in the output circuit of the second Amentioned gas tube.

' 5. In photo radio systems, a scanner producing picture modulations, a pair of gas trlodes connected together, one normally conducting and one normally blocked, a saw-tooth generator, means to combine the scanner modulation with the output of the saw-tooth generator in the input of the blocked gas triode whereby it fires at the time in each cycle of the saw-tooth voltage that the sum of the amplitudes of the modulation and saw-tooth'waves in saidcycle rst reach the firing voltage, means for blocking the previously conducting gas triode when the other triode fires, and means to reblock, the last mentioned gas triode and retire the first-mentioned gas triode before the end of the cycle in which the blocked triode was red, and means to transmit a signal corresponding to the pulse produced by the normally blocked gas triode when it wa fired.

6. In photo radio systems, 'a scanner producing picture modulations, aY pair oi.' gas triodes con"- nected together, one normally conducting and one normally blocked, a sawtooth generator means to combine the scanner modulations with the output of the saw-'tooth generator in the input of the blocked` gas triode whereby 1t mes at the time in each cycle of the saw-tooth voltage that the sum of the amplitudes of the modulations and saw-tooth waves in said cycle first reach the ringvoltage, means for-blockingthe pulse, producing a periodic voltage having said constant frequency, combining the said voltage pulses and periodic voltage lto produce a result- 7. In photo radio systems, a circuit adapted to contain phased pulses representing signal modulations, an electric valve, a condenser connected to said circuit through said valve, and means to close a conducting circuit around said`condenser when a signal pulse is received and to open said circuit when a signal pulse is not received whereby the condenser can receive a charge from a pulse greater in value than the preceding pulse and can release a portion of its charge through said conducting circuit when a pulse is received of less value than the preceding pulse, and a picture recorder having connections to said condenser to produce a record corresponding to the variations of potential at the terminals of the condenser.

8. In photo radio systems for receiving phase modulations, a triode having a blocking bias on its control electrode, means for applying the phase modulations to thel input circuit of said tube, va condenser connected across the output circuit of said tube, an electric valve having its cathode connected to said condenser and its anode to one terminal of said means whereby the. condenser receives a charge from each signal pulse higher than the previous pulse and discharges through the plate circuit of said tube on receipt of a pulse smaller than the previous pulse, and a picture recorder having connections to said condenser.

9. In photo radio systems, a circuit adapted to contain phased pulses representing signal modulations, an amplifier having its input cir' cuit connected to the mst-mentioned circuit, a gas triode having its input connected to the output of said amplier, a second gas triode connected to the first gas triode, the rst gas triode being normally non-conducting and the second gas triode being normally conducting, means connected to the amplifier to fire the first gas tube on arrival of a signal pulse, means to render the second gas triode non-conducting when the first gas triode becomes conducting and means energized by the output of the first gas triode to reduce the amplification of said amplier to zero.

10. The method of communicating by signal characters' transmitted at various phases of a predetermined frequency, which consists in re-' ceiving said signal characters, producing a constant voltage between adjacent pulses varying in amplitude with the phase of the initial one thereof and producing a record by the last-mentioned voltage.

11. 'I'he method of communicating by signal characters transmitted at various phases of a predetermined frequency, which consists in receiving said signal characters, producing thereby short voltage pulses at the start only of each received character to eliminate multipath addiant voltage, producing a constant voltage between adjacent pulses having an amplitude that varies with the phase of the initial one thereof and'producing a record by the last-mentioned voltage.

13. The method of communicating by short electrical pulses of uniform length transmitted at various phases of ya constant frequency to indicate a signal, -which consists in receiving the electrical pulses, producing -a periodic voltage having said constant frequency, producing an additional voltage from said pulses, blocking spurious additions to the received pulses by said additional voltage, and removing the additional voltage by the peak .of said periodic voltage in each cycle thereof.

14. The method of transmitting signal characters of constant frequency and variable length, which consists in generating a periodic voltage ofsaid constant frequency, transmitting a shorter electrical pulse at one end only of each of'said characters at a phase of said frequency varying with the length of the characters, receiving said electrical pulses, producing a second' periodic voltage having said constant frequency, producing an additional voltage upon receipt of each pulse, blocking spurious additions to each of the received pulses by said additional voltage', and thereafter removing the additional voltage by the peaks of said periodic voltage.

15. The method of transmitting signal characters of constant frequency and variable length,

. which consists in generating a periodic voltage of said constant frequency, transmitting a shorter electrical pulse at one end only of each of said characters at a phase'of said frequency varying with the length of the characters, receiving said electrical pulses, producing an additional voltage upon receipt of each pulse, and blocking spurious additions to each of the received pulses by said additional voltage.

16. In a signaling system, a signaling device, a pair of tubes having input and output circuits, means for producing less than a blocking voltage in the input circuit of one tube and a blocking voltage in the input circuit of the other tube, whereby one tube is normally conductive and the other normally blocked, a generator of altemating voltages, means 'for combining the output voltages of said device and generator in the input circuit of the normally blocked tube, the output voltages of said device and generator in a half voltage cycle separately being less, and combined being more, than that required to render the normally blocked tube conductive, means for blocking the normally conductive tube when the normally blocked tube becomes conductive, and means for rendering the last-mentioned tube non-conductive before the end of said half cycle.

17. In a signaling systema signaling device, a pair of tubes having input and output circuits, means for producing less than a blocking voltage in the input circuit of one .tube and a blocking voltage in the input circuit of the other tube, whereby one tube is normally conductive and the other normally blocked, a generator of alo ternating voltages, means for combining the outa half voltage cycle separately being less, and

combined being more, than that required to render the normally blocked tube conductive, means for blocking the normally conductive tubeA when the normally blocked tube becomes conductive, means for rendering the last-mentioned tube nonconductive and the other tube conductive immediately afterLit is rendered conductive by said combined voltages, and means for preventing the last-mentioned tube from becoming conductive a second time in a half cycle.

18. In a signaling system, a signaling'device, a pair of tubes having input and outputcircuits, means for producing less than a blocking voltage in the input circuit of one tube and a blocking voltage in the input circuit oi the other tube, whereby one tube is normally conductive and the other normally blocked, a saw-tooth generator, means for combining the output voltages of said device and generator in the input circuit of the normally blockedtube, the output voltages of saidvdevice and generator in a half saw-tooth voltage cycle separately being less, and combined being more, than that required to render the normally blocked tube conductive, means i'or blocking the normally conductive tube when the normally blocked tube becomes conductive, and means for rendering the last-mentioned tube Y non-conductive before the end of said half cycle.

19. In a signaling system, a signaling device, a pairof tubes having inputand output circuits, means for producing less than a blocking voltage in the input circuit of one tube and a blocking voltage in that of the other tube, whereby one tube is normally conductive and the other -ncrmally blocked, a saw-tooth generator, means for combining the output voltages of said device and generator in the input circuit of the normally blocked tube, the output voltages of said device and generator in a half saw-tooth voltage cycle separately being less, and combined being more, than that required to render the normally blocked tube conductive, means for blocking the. norpair oi gas tubes, means for producing a iiring l bias voltage in the input circuit of one tube and less than firing bias voltage in the input circuit mally conductive tube when thenormally blocked tube becomes conductive, means for rendering the last-mentioned tube non-conductive and the other tube conductive immediately after it isy rendered conductive by said combined voltages, and means for preventing the last-mentioned tube from becoming conductive a second time in a half cycle.

. 20. In a signaling system, a signaling device, a

normally blocked gas tube, a generator of alternating voltage, means for combining the output voltages o said .device and generator in theinput circuit of said gas tube, the output voltages cf of the other tube, whereby one is normally conductive and the other is'normally quenched. av

ductive gas tube when the normally quenched gas tubeis fired by said combined voltages, and means to prevent the last-mentioned gas tube from ilring a second time in said cycle.

24. In a signaling system, a signaling device,

a pair of gas tubes, means for producing a ring bias voltage in the input circuit of one tube and less than firing bias voltage in the input circuit of the other tube, whereby one is normally conductive and theother isr normally quenched, a saw-toothed generator, means-for combining the output voltages of the signaling device and generator in the inpui'I circuit of the normally quenched gas tube, the output voltages of said -device and generator in each saw-tooth voltage cycle separately beinggless, and combined being more, than the iring'; voltage of the normally quenched tube, means for quenching the normally conductive gas tube when the normally quenched gas tube is fired bysaid combined voltages, and means to prevent the 'last-mentioned gas tube from ring a second time in said cycle.

25. Ina signaling system, e.` signaling device, a pair of gas tubes, means for producing a firing bias voltage in the input circuit of one tube and less than firing bias voltage in the input circuit of the other tube, whereby one is normally conductive and the other is normally quenched, a saw-tooth generator, means for combining the output voltages of said signaling device and generator in the input circuit of the normally quenched gas tube, the output voltages of said device and generator in a saw-tooth voltage cycle separately being less, and combined being more, than the firing voltage of the normally quenched tube, means for quenching the normally conductive gas tube when `the normally quenched gas tube is fired by said combined voltages, means to prevent the last-mentioned cuit of said gas tube, the output voltages of said scanner and generator in a saw-tooth half cycle separately being less, and combined being more, than that required to re'said tube, means for quenching said gas tube immediately after tiring, and means to prevent said gas tube froml tiring a second time in said half cycle.

22. In a photo radio system. a picture scanner, v

gas tube from firing a second time in said cycle, and means for transmitting the pulses produced in the output circuit of the normally quenched tube.

26. In photo jradio systems, a picturescanner, a pair of gas tubes, means for producing a'firing bias voltage in the input circuit of one tube and less than .tiring bias voltagejin the input circuit of the other gas tube, whereby one is normally conductive and the other isy normally quenched, a generator of alternating voltage, means for combining the scanner and generator output voltages in 'the input circuit of the normally quenched gas tube, the output voltages of said scanner and generator in a voltage cycle separately being less, and combined being more, than the iiring voltage of the normally quenched tube, means for quenching thenormally conductive gas tube when the normally quenched gas tube is fired by said combined voltages, and means to prevent the last-mentioned gas tube from iiring a second time in said cycle.

27. In photo radio systems, a picture scanner,

l a pair of gas tubes, means for producing a iiring bias voltage in the input circuit oi one tube and less than iiring bias voltage in the input circuit of the other gas tube, whereby one is normally conductive and the other; is normally quenched, a generator of alternating voltage, means for combining the scanner and generator output voltages inthe input circuit of the normally quenched gas tube, the output voltages of said scanner and generator in a voltage cycle separately being less, and combined being more;

less than firing bias voltage in the input 'circuit of the other-gas tube, whereby one is normally conductive and the other is normally quenched, a generator of saw-tooth voltage, means for combining the scanner and generator output volttages in the input circuit of the normally quenched gas tube, the 'output voltages of said scanner and generator in a saw-tooth voltage cycle separately being less, and combined being more, than the iiring voltage of the normally quenched tube, means for quenching the normally conductive gas tube when the normally quenched gas tube is fired by said combined voltages, and means to prevent the last-mentioned gas tube from firing twice in said cycle.

i 29. In photo radio systems, a picture scanner, a pair of gas tubes, means for producing a iiring bias voltage in the input circuit of one tube and lessV than firing bias voltage in the input circuit of the other gas tube, whereby one is normally conductive and the other is normally quenched, a generator of saw-tooth voltage,vmeans for combining the scanner and generator output voltages in the input circuit of the normally quenched gas tube, the output voltages of said scanner and generator in a saw-tooth voltage cycle separately .being less, and combined being a normally quenched gas tube having its input circuit connected to the output circuit of said amplifier, said gas tube being adapted to be fired by the amplified pulses, means energized by the output of said tube for producing a reduction of the voltage of the amplified pulses below `connected to the output circuit of said amplier, said gas tube being adapted to be fired by the amplified pulses, a condenser charged by the output of said tube, means controlled by the charge of said condenser for producing a reduction of the voltage of the ampliiied pulses below the iiring voltage of the said gas tube, an alternating current generator having said frequency,

and means controlled by the voltage `of said generator at a predetermined phase point in each cycle for discharging said condenser before the start of the next cycle.

32. In a receiving system for signal pulses transmitted in timed relation with a predetermined frequency, a circuit adapted to contain said pulses, a gas tube, means for producing a blocking voltage in the input circuit of said gas tube, an amplifier having its input circuit vconnected to the first-mentioned circuit and its output circuit connected to the input circuit of said gas tube, the amplified pulses being adapted to fire said gas tube, a second gas tube, means for producing a iiring bias in the input circuit of the' second gas tube, means connected to said gas tubes for quenching one tube whenthe other one fires, means energized by the output of the first gas tube for producing a reduction of the voltage of the amplified pulses below the iiring voltage of the first gas tube, an alternating current generator having said frequency, and means controlled by the voltage of said generator at a predetermined phase point in each cycle for annul-r ling said reduction of voltage until the start of the next cycle.

33. In a receiving system for signal pulses transmitted in timed relation with a predetermined frequency, a circuit adapted to contain said pulses, a gas tube, means for producing a blocking voltage in the input circuit of said gas tube, an amplifier having its input circuit connected to the first-mentioned circuit and its output circuit connected to the input circuit oi' said gas tube, the amplified pulses being adapted to fire said gas tube, a second gas tube, means for producing a iiring bias in the input circuit of the second gas tube, means connected to said gas tubes for quenching one tube when the other one fires, a condenser charged by the output of the first gas tube for producing a reduction of the' charging said lcondenser before the start of the l next cycle.

34. In a -receiving system for signal pulses transmitted in varying phase relation with respect to a predetermined frequency, a signal source adapted to contain said pulses, an im of said condenser for recording signal elements.

35. In a receiving system for signal pulses transmitted in varying phase relation with respect to a predetermined frequency, a signal sourceadapted to contain said pulses, an impedance and an electric valve connected in series conducting relation across said signal source, a condenser connected across a part of said im- `edance, a saw-tooth generator having said frequency, means for combining the voltages of said signal pulses with the voltage of said generator, means for varying a part of said impedance `with the variations of said combined voltages, and means controlled by the voltage of said condenser for recording signal elements.

36. In a receiving system for signal pulses transmitted in varying phase relation with a predetermined frequency, a signal source adapted to contain said pulses, a vacuum tube, means for biasing said tube substantially to cut olf, an

'electric valve connected in a series circuit with the internal plate impedance of said vacuum tube in conducting relation across said signal source, a condenser connected across a portion of said circuit, a triangular wave generator having said frequency, means for combining the voltage of electric valve connected in a series circuit with the internal plate impedance of said vacuum tube in conducting relation across said signal source, a condenser connected across a portion of said circuit, a saw-tooth generator having said frequency, means for combining the voltage of the signal pulses with the voltage of said generator in the input circuit of said vacuum tube, and means controlled by the voltage of said condenser for recording signal elements having shade densities depending upon the charge in said condenser.

38. In a receiving system for .signal pulses transmitted in varying phase relation with respect to a predetermined frequency, a signal source adapted to contain said pulses, a vacuum tube, means for normally blocking said tube, an electric valve connected in a s-eries conducting circuit with the internal plate impedance of said vacuum tube in conducting relation across said signal source, a condenser connected across said plate impedance, a triangular wave generator having said frequency, means for combining the voltage of said signal pulses with the voltage of one half cycle of said generator in the input circuit of said vacuum tube, the voltage of said generator alone being less, and combined with said pulses being more, 'than thatrequired to unblock said vacuum tube, and means'controlled by the voltage of said condenserfor recording signal elements having shade densities depending upon the charge in said condenser,

39. In a receiving system for signal pulses transmitted in varying phase relation with re spect to a predetermined frequency, a signal source adapted to, contain said pulses, a Vacuum tube, means for normally blocking said tube, an electric valve connected in a series circuit with the internal plate impedance of said vacuum tube in conducting relation across said signal source, a condenser connected across said plate impedance, a saw-tooth generator havingsaid frequency, means for combining the voltage of the signal pulses with the voltage of said generator inthe input circuit of said vacuum tube, the voltage'of said generator alone being less, and combined with said pulses being more, than that required to unblock said vacuum tube, and means controlled by the voltage of said condenser for' recordingsignal elements having shade densities depending upon the charge in said condenser.

40. In a photo radio receiving system for signal pulses transmitted in varying phase relation with a predetermined frequency, a signal source adapted to contain said pulses, a vacuum tube, means for biasing said tube substantially to cut off, an electric valve connected in a series circuit with the internal plate impedance of said vacuum tube in conducting relation across said signal source, a condenser connected across said plate impedance, a saw-tooth generator having said frequency, means for combining the voltage of the signal pulses with the voltage of said generato in the input circuit of said vacuum tube, the voltage of said generator alone being'less, and combined with said pulses being more, than that requiredto unblock said vacuum tube, means for lsupporting a recording surface, a recording unit,

means for causing said unit to scan said surface in unison with said frequency, and means for plate impedance, a saw-tooth generator having D said frequency, means for combining the voltage of the signal pulses with the voltage of said generator in the input circuit of said vacuum, tube, the voltage of said generator alone being less, and combined with said pulses being more,

than that required to unblock said vacuum tube, means for supporting a light sensitive surface, a recording unit, means for causing said unit to scan said surface in` unison with said frequency, and means for projecting light-from said unit onto said surface that varies with the voltage of said condenser. A

42.'In photo radio systems, a circuit adapted to contain phased signal pulses transmitted in timed relation with a predetermined frequency, means for holding a record. shee t, recording means for producing a record corresponding to said pulses by scanning lines across said sheet, a generator of alternating current controlling the speed of the scanning movement of said recording means, means controlled by the signal pulses for a signal source controlling the phase and frequency of said generator, and means for switching the signal pulses to the last-mentioned means during the time of]` passage between the end of one line and the beginning of another and to said recording means at all other times.

43. In photo radio systems, a circuit adapted to contain phased signal pulses transmitted in timed relation with a predetermined frequency, means for holding a record sheet, recording means for producing a record corresponding to said pulses by scanning lines across said sheet, an oscillator having frequency control members adjusted to oscillate at substantially said frequency, means for switching the signal pulses to said members during the time of passage of the recording means between the end of one line and the beginning of another and to said recording means at all other times, and means for controlling the speed of the scanning movement of said recording means by the frequency and phase of said oscillator.

44. lIn photo radio systems, a circuit adapted to contain phased signal pulses transmitted in timed relation with a predetermined frequency, a drum having a clamp on`its surface for holding a record sheet, a motor operating said'drum, recording means for producing a record on said sheet cor. responding to. said pulses, an oscillator having frequency control members adjusted to oscillate at substantially said frequencyfmeans for switching the signal pulses to said members when the recording means is passing over said clamp and to said recording means when it is passing over the remaining surface of said drum, and means for controlling the speed of said motor by the frequency and phase of said oscillator.

45. In photo radio systems, a circuit adapted to containphased signal pulses transmitted in timed relation with a predetermined frequency, a drum having a clamp on its surface for holdinga recording sheet, a motor operating said drum, recording means for producing a record on said sheet corresponding to said pulses, a generator of alternating current controlling the speed of said motor, means controlled by said signal pulses for controlling the phase and frequency -of said generator, and means for switching the signal pulses to the lastfmention'ed means when the recording means is passing over said clamp and to said recording means when it is passing over the remaining surface of said drum.

RICHARD E. MATHES. WARREN H. BLISS. 

